Direct Hydroxylation of Benzene to Phenol Over TS-1 Catalysts

catalysts

Article Direct Hydroxylation of Benzene to Phenol over TS-1 Catalysts

Yuecheng Luo, Jiahui Xiong, Conglin Pang, Guiying Li * and Changwei Hu * Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China; [email protected] (Y.L.); [email protected] (J.X.); [email protected] (C.P.) * Correspondence: [email protected] (G.L.); [email protected] (C.H.); Tel./Fax: +86-28-8541-5745 (G.L.); +86-28-8541-1105 (C.H.)

Received: 2 January 2018; Accepted: 22 January 2018; Published: 26 January 2018

Abstract: We synthesized a TS-1 catalyst to directly hydroxylate benzene to phenol with H2O2 as oxidant and water as solvent. The samples were characterized by FT-IR (Fourier Transform Infrared), DR UV-Vis (Diffused Reﬂectance Ultraviolet Visible), XRD (X-ray diffraction), SEM(scanning electron microscope), TEM (Transmission Electron Microscope), XPS (X-ray photoelectron spectroscopy), ICP (inductively coupled plasma spectrum), and N2 adsorption-desorption. A desirable phenol yield of 39% with 72% selectivity was obtained under optimized conditions: 0.15 g (0.34 to the mass of benzene) TS-1, 5.6 mmol C6H6, reaction time 45 min, 0.80 mL H2O2 (30%), 40.0 mL H2O, and reaction temperature 70 ◦C. The reuse of the TS-1 catalyst illustrated that the catalyst had a slight loss of activity resulting from slight Ti leaching from the ﬁrst run and then kept stable. Almost all of the Ti species added in the preparation were successfully incorporated into the TS-1 framework, which were responsible for the good catalytic activity. Extraframework Ti species were not selective for hydroxylation.

Keywords: benzene; hydroxylation; phenol; TS-1 catalyst

1. Introduction Phenol is an important chemical widely used in medicine, pesticides, plastics, and so on. The cumene method is the main process for phenol production in current commercial manufacturing, and this method has three steps from benzene to the target product [1–3]. The yield of phenol is not high, with disadvantages of high energy consumption and equipment corrosion. There is also an equimolar amount of acetone formed as a co-product [4,5]. The number of studies on the synthesis of phenol directly from benzene has been increasing, emphasizing environmental friendliness and economic feasibility. The one-step direct hydroxylation of benzene to phenol with hydrogen peroxide, which requires the activation of C–H bonds in the aromatic ring and the subsequent insertion of oxygen, has drawn much attention [6–9]. Several efficient catalysts with high activity and stability have been designed and tested [10–13]. TS-1, a Ti-containing MFI (Mordenite Framework Inverted) structure zeolite, first synthesized in 1983 by Taramasso et al. [14], has been found to display excellent catalytic activity in the oxidation of the aromatic ring, such as aromatic hydroxylation and cyclohexanone ammoximation [15–18]. A TS-1-catalyzed reaction can proceed under mild conditions with aqueous H2O2 as the oxidant so as to satisfy the demand of environmental protection. TS-1 suffers from an intracrystalline diffusion limitation on account of the micropores blocked by the reaction intermediates and by-products, and this limitation is most pronounced for a low temperature liquid phase reaction [19,20]. Thereby, research on TS-1 post-synthesis to extend the pore size has attracted much attention, where the pore-modified TS-1 catalyst may possess the advantage of a micro/mesoporous structure. Hierarchical zeolites possess

Catalysts 2018, 8, 49; doi:10.3390/catal8020049 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 49 2 of 14 both the typical characteristics of zeolites and the properties usually related to amorphous materials, such as a crystalline structure and the presence of larger pores [21]. The practical applications of hierarchical zeolites have been greatly extended, and have become one of the excellent potential strategies to overcome the mass transport limitation in the catalyst [22]. In our previous work, an activated carbon catalyst was found to be efﬁcient for the direct hydroxylation of benzene [23,24]. However, the repeated use of the activated carbon catalyst requires cumbersome treatments, and the yield of phenol was not high enough. The Fe-loaded ZSM-5 catalyst, with N2O as the oxidant in a hydroxylation reaction, usually had an excellent selectivity to phenol [25]. On hierarchical zeolite material post-treated ZSM-5, N2O oxidated benzene to phenol with good stability and selectivity [26]. There are some reports of benzene hydroxylation by a Pd-loaded membrane reactor, with H2 and O2 gases as co-reactants, which had good selectivity [27–29]. N2O as an oxidant requires a high reaction temperature, and O2 as an oxidant usually requires inert gas as a carrier. Therefore, H2O2 as a cheap and energy-saving oxidant is widely used in current research. Liya Hu et al. reported that vanadium-containing mesoporous carbon composites displayed superb catalytic activity, but the reuse was not good enough [30]. In the hydroxylation of benzene by the TM PQ Corporation commercial catalyst TS-PQ , the conversion of H2O2 was 19.4% and the selectivity of the phenol was 92% with an excess use of benzene [31]. The Fe-CN/TS-1 catalysts, with metal chloride as a precursor and a TS-1 zeolite as a support, could oxidize benzene to phenol in a biphasic water–H2O2/acetonitrile medium with visible light irradiation [32,33]. Therefore, the preparation of a catalyst with high catalytic activity and high stability is of great interest. In the present work, a TS-1 catalyst was synthesized with a modiﬁed method and used for benzene hydroxylation to phenol, with H2O2 as the oxidant and water as the solvent. Commercial TS-1 and S-1 were also used for comparison.

2. Results and Discussion

2.1. Catalytic Activity Test

2.1.1. Optimization of Reaction Conditions The effect of reactant concentration. The concentration of hydrogen peroxide was investigated, and the results are shown in Figure1a. With an increasing concentration of H 2O2, the yield of phenol (PH) increased, reached the maximum at the concentration of 0.20 mol/L, and then decreased. The yield of catechol (CAT), hydroquinone (HQ), and benzoquinone (BQ) monotonously increased. The high concentration of H2O2 led to the over-oxidation of phenol to dihydroxybenzene (DHB, namely, CAT and HQ) and benzoquinone [34], so the selectivity of the phenol did not change much when the concentration of H2O2 was less than 0.20 mol/L. Then, the selectivity of the phenol decreased gradually for the production of more DHB and benzoquinone. The effect of reaction temperature. The activity of the catalysts was tested at 50 ◦C, 60 ◦C, 70 ◦C, 80 ◦C, and 90 ◦C, and the results are shown in Figure1b. With an increasing reaction temperature, the yield of phenol enhanced, reached a maximum at 70 ◦C, and then decreased, whereas the yield of DHB increased monotonously. It has been reported that a low temperature is beneﬁcial to restrain the over-oxidation of phenol [35,36]. When the reaction temperature reached 80 ◦C, the generation of by-products was signiﬁcantly faster than the generation of phenol, which led to an obvious decrease of selectivity to phenol. The effect of reaction time. The reaction time was studied in the range of 25–65 min and the results are shown in Figure1c. The conversion of benzene and the yields of DHB and benzoquinone increased monotonically with time. However, the yield of phenol increased within 45 min and then decreased, and the selectivity of phenol was stable within 45 min and then decreased. A longer reaction time resulted in an increased formation of over-oxidation products [37], which reduced the yield and selectivity to phenol. Catalysts 2018, 8, 49 3 of 14

CatalystsThe 2018 effect, 8, x of FOR catalyst PEER REVIEW amount. The mass ratio of TS-1 to benzene was increased from 0.30 to30.39, of 14 and the results are shown in Figure1d. The yield of phenol increased with the mass ratio of TS-1 to benzeneup to 0.38, up and to 0.38,the yield and theof over yield-oxidation of over-oxidation products, products, DHB and DHB benzoquinone, and benzoquinone, had the same had thevariation same variationtrend. When trend. the mass When ratio the of mass TS-1 ratio to benzene of TS-1 was to benzene greater than was 0.39, greater the thanexcess 0.39, catalyst the excessmight catalyze catalyst the noneffective decomposition of H2O2, which was responsible for the decrease of benzene conversion. might catalyze the noneffective decomposition of H2O2, which was responsible for the decrease of benzeneConsidering conversion. the yield Considering and selectivity the of yield phenol, and selectivitya mass ratio of of phenol, TS-1 to a massbenzene ratio of ofaround TS-1 to 0.34, benzene namely of around0.15 g TS 0.34,-1, was namely rational. 0.15 g TS-1, was rational.

Figure 1. The yield of products, the conversion of benzene, and the selectivity of phenol. Reaction Figure 1. The yield of products, the conversion of benzene, and the selectivity of phenol. Reaction conditions: 5.6 mmol C6H6, 40.0 mL H2O: (a) The effect of concentration of H2O2, mTS-1/mbenzene = 0.34, conditions: 5.6 mmol C6H6, 40.0 mL H2O: (a)◦ The effect of concentration of H2O2, mTS-1/mbenzene = 0.34, reaction time 45 min, reaction temperature 70 C; (b) The effect of reaction temperature, mTS-1/mbenzene reaction time 45 min, reaction temperature 70 °C ; (b) The effect of reaction temperature, mTS-1/mbenzene = 0.34, reaction time 45 min, 0.80 mL H2O2 (30%); (c) The effect of reaction time, mTS-1/mbenzene = 0.34, = 0.34, reaction time 45 min, 0.80 mL H2O2 (30%);◦ (c) The effect of reaction time, mTS-1/mbenzene = 0.34, 0.80 mL H2O2 (30%), reaction temperature 70 C; (d) The effect of catalyst amount, 45 min, 0.80 mL 0.80 mL H2O2 (30%), reaction temperature◦ 70 °C ; (d) The effect of catalyst amount, 45 min, 0.80 mL H2O2 (30%), reaction temperature 70 C. H2O2 (30%), reaction temperature 70 °C . 2.1.2. Recycling of the Catalyst 2.1.2. Recycling of the Catalyst The stability of TS-1 was tested and the results are shown in Figure2. After reaction, the catalyst The stability of TS-1 was tested and the results are shown in Figure 2. After reaction, the catalyst was ﬁltrated and calcined at 550 ◦C for 10 h. The yield of phenol and the conversion of benzene was filtrated and calcined at 550 °C for 10 h. The yield of phenol and the conversion of benzene reduced slightly from the ﬁrst run to the second, then kept almost invariable in the following runs. reduced slightly from the first run to the second, then kept almost invariable in the following runs. The selectivity of phenol kept almost constant in all runs. The selectivity of phenol kept almost constant in all runs. According to the above results, the optimal conditions were the following: 5.6 mmol C6H6, ◦ 0.80 mL H2O2 (30%), 40.0 mL H2O, reaction temperature 70 C, reaction time 45 min, and 0.15 g TS-1. Under the aforementioned optimized conditions, we obtained a phenol yield of about 39%, with a selectivity of about 72%, and the turn-over frequency (TOF) value reached 73 h−1, which indicated the good catalytic performance of the TS-1 catalyst in the hydroxylation of benzene. As a comparison, we used commercial TS-1 as a catalyst for the hydroxylation of benzene. We also prepared an S-1 catalyst by the same strategy as for the TS-1 catalyst, with no titanium trichloride addition. The catalytic activity of commercial TS-1 and S-1 was tested under the optimized conditions. The yield of phenol was 17% and the selectivity of phenol was 75% over the commercial TS-1, whereas the yield of phenol over S-1 was low, less than 1%. This indicated that Ti is essential for the hydroxylation reaction, and that the TS-1 prepared in the present work exhibited high activity [38–40].

Figure 2. The stability of catalyst. Reaction conditions: mTS-1/mbenzene = 0.34, 40.0 mL H2O, 5.6 mmol C6H6, reaction time 45 min, 0.80 mL H2O2 (30%), reaction temperature 70 °C .

Catalysts 2018, 8, x FOR PEER REVIEW 3 of 14 up to 0.38, and the yield of over-oxidation products, DHB and benzoquinone, had the same variation trend. When the mass ratio of TS-1 to benzene was greater than 0.39, the excess catalyst might catalyze the noneffective decomposition of H2O2, which was responsible for the decrease of benzene conversion. Considering the yield and selectivity of phenol, a mass ratio of TS-1 to benzene of around 0.34, namely 0.15 g TS-1, was rational.

Figure 1. The yield of products, the conversion of benzene, and the selectivity of phenol. Reaction

conditions: 5.6 mmol C6H6, 40.0 mL H2O: (a) The effect of concentration of H2O2, mTS-1/mbenzene = 0.34, reaction time 45 min, reaction temperature 70 °C ; (b) The effect of reaction temperature, mTS-1/mbenzene = 0.34, reaction time 45 min, 0.80 mL H2O2 (30%); (c) The effect of reaction time, mTS-1/mbenzene = 0.34,

0.80 mL H2O2 (30%), reaction temperature 70 °C ; (d) The effect of catalyst amount, 45 min, 0.80 mL H2O2 (30%), reaction temperature 70 °C .

2.1.2. Recycling of the Catalyst The stability of TS-1 was tested and the results are shown in Figure 2. After reaction, the catalyst was filtrated and calcined at 550 °C for 10 h. The yield of phenol and the conversion of benzene reduced slightly from the first run to the second, then kept almost invariable in the following runs. Catalysts 2018 8 The selectivity, , 49of phenol kept almost constant in all runs. 4 of 14

Catalysts 2018, 8, x FOR PEER REVIEW 4 of 14

According to the above results, the optimal conditions were the following: 5.6 mmol C6H6, 0.80 mL H2O2 (30%), 40.0 mL H2O, reaction temperature 70 °C , reaction time 45 min, and 0.15 g TS-1. Under the aforementioned optimized conditions, we obtained a phenol yield of about 39%, with a selectivity of about 72%, and the turn-over frequency (TOF) value reached 73 h−1, which indicated the good catalytic performance of the TS-1 catalyst in the hydroxylation of benzene. As a comparison, we used commercial TS-1 as a catalyst for the hydroxylation of benzene. We also prepared an S-1 catalyst by the same strategy as for the TS-1 catalyst, with no titanium trichloride addition. The catalytic activity of commercial TS-1 and S-1 was tested under the optimized conditions. The yield of phenol was 17% and the selectivity of phenol was 75% over the commercial TS-1, whereas the yield of phenol over S-1 was low, less than 1%. This indicated that Ti is essential for the hydroxylation reaction, and that the TS-1 prepared in the present work exhibited high activity FigFigureure 2 2.. TThehe stability of catalyst.catalyst. ReactionReaction conditions:conditions: m mTS-1TS-1/m/mbenzenebenzene = 0.34= 0.34,, 40.0 40.0 mL mL H H2O,2O, 5.6 5.6 mmol mmol [38–40]. ◦ CC6H6H6, 6reaction, reaction time time 45 45 min, min, 0.80 0.80 mL mL H H2O2O2 (30%),2 (30%), reaction reaction temperature temperature 70 70 °C .C.

2.2.2.2. Characterization of the Catalyst

2.2.2.1.2.1. FT FT-IR-IR Spectroscopy The FT FT-IR-IR spectra of TS-1,TS-1, used TS TS-1,-1, and commercial TS TS-1-1 are shown in Figure 33.. BasedBased onon thethe literature [21,41 [21,41––45]45],, the band at 550 550 cm cm−−11 waswas assigned to the asymmetric framework vibration of double five ﬁve rings rings and and the the characteristic characteristicss of of the the MFI MFI structure, structure, and and the the band band at at 965 965 cm cm−1 −was1 was evidence evidence of theof the isomorphous isomorphous substitution substitution of Ti of in Ti inthe the TS TS-1-1 catalyst catalyst framework. framework. As Asshown shown in Figure in Figure 3, 3the, the ban bandsds at 553at 553 cm cm−1 and−1 and 965 965cm− cm1 demonstrate−1 demonstrate the successful the successful incorporation incorporation of Ti into of Ti the into MFI the framework MFI framework of both of theboth TS the-1 catalyst TS-1 catalyst and the and commercial the commercial TS-1. TS-1.There There was no was change no change in the in characteristic the characteristic peaks peaks of theof used the TSused-1 comparing TS-1 comparing with the with parent the parent TS-1. TS-1.

FigFigureure 3 3.. FTFT-IR-IR spectra of TS TS-1,-1, used TS TS-1,-1, and commercial TS TS-1.-1.

2.2.2.2.2.2. Diffuse Reflectance Reﬂectance UV UV-Vis-Vis The diffuse reflectance reﬂectance ( (DR)DR) UV UV-Vis-Vis spectra of the TS-1,TS-1, used TS TS-1,-1, and commercial TS TS-1-1 are shown in Figure Figure 44.. BasedBased onon thethe literatureliterature [[4646––48]48],, the the absorption absorption band band at at around around 210 210 nm was the characteristic peak of Ti tetrahedrally incorporated into the framework, and the absorption band at 26 260–3300–330 nm was assigned to penta-penta- andand hexa-coordinatedhexa-coordinated andand anataseanatase extraframework extraframework Ti Ti species. species. As As shown shown in in Figure Figure4, 4,the the presence presence of of the the band band at at around around 210 210 nm nm and and the the absence absence ofof bandsbands aroundaround 260–330260–330 nm proved that almost all of the Ti species were incorporated in the framework of TS-1.TS-1. The commercial TS TS-1-1 had an obvious absorption band at 26 260–3300–330 nm, which implied the existence of penta penta-- and hexa-coordinatedhexa-coordinated Ti species and anatase TiO2. The framework Ti species had a good catalytic performance; meanwhile, Ti species and anatase TiO2. The framework Ti species had a good catalytic performance; meanwhile, the extraframework Ti species were usually responsible for the decomposition of H2O2 [49,50]. From the DR UV-Vis spectra of used TS-1, no obvious change of absorption bands was observed.

Catalysts 2018, 8, 49 5 of 14

the extraframework Ti species were usually responsible for the decomposition of H2O2 [49,50]. From theCatalysts DR 2018 UV-Vis,, 8,, xx FOR spectraFOR PEERPEER of REVIEWREVIEW used TS-1, no obvious change of absorption bands was observed. 5 of 14

FigureFigure 4. DiffuseDiffuse reflectance reﬂectance ( (DR)DR)) UV UV-Vis--Vis spectraspectra ofof TS-1,TS--1, used TS-1,TS--1,, andand commercialcommercial TS-1.TS--1.

2.2.3.2.2.3..3. X X-ray--ray Diffraction The XRD XRD patterns patterns of of TS TS-1,--1, used used TS TS-1,--1, S--1 S-1,,, and and commercial commercial TS--1 TS-1 are areshown shown in Figure in Figure 5. The5. Thecharacteristic characteristic diffraction diffraction peaks peaks at 2θ at of 2 θ7.9°,of 7.9 8.8°,◦, 8.8 23.1°,◦, 23.1 23.9°,◦, 23.9 and◦, 24.4° and 24.4 proved◦ proved that the that TS the--1, TS-1, S--1,, and S-1, andcommercial commercial TS--1 TS-1 had had a well a well-crystallized--crystallized MFI MFI structure structure [51[51 [51–55]–55.. ].NoNo No significantsignificant signiﬁcant diffractiondiffraction diffraction peakpeak peak was was observed at 2θ of 25.4°◦ for all of the samples, which indicated the absence of crystalline TiO22. observed at at 2 θ 2θof of 25.4 25.4°for allfor of all the of samples, the samples, which which indicated indicated the absence the of absence crystalline of crystalline TiO2. Although TiO . theAlthough DR UV-Vis the DR spectra UV-- ofVis commercial spectra of TS-1,commercial shown inTS Figure--1, shown5, showed in Figure the presence 5, showed of extraframeworkthe presence of extraframework TiO22 species, the XRD patterns with no obvious◦ peak at 25.4° indicated that the TiOextraframework2 species, the XRD TiO patternsspecies, with the XRD no obvious patterns peak with at 25.4no obviousindicated peak that at the 25.4° extraframework indicated that TiO the2 speciesextraframework might be highlyTiO22 species dispersed might with be a highly very small dispersed size in with commercial a very TS-1small [56 size]. The in commercial highly dispersed TS--1 [56]. The highly dispersed TiO22 increased the contact with H22O22, increasing the decomposition of TiO[56]2. increasedThe highly the dispersed contact with TiO H in2Ocreased2, increasing the contact the decomposition with H O , increasing of H2O2 [ 50the]. Thisdecomposition might be the of reasonH22O22 [[50 why]].. ThisThis the commercialmightmight bebe thethe TS-1 reasonreason had why inferiorwhy thethe catalytic commercial activity TS-- in1 had the hydroxylationinferiorinferior catalyticcatalytic of benzene activityactivity underinin thethe thehydroxylation conditions testedof benzene in the under present the work. conditions From tested the XRD in the patterns present of work. the used From catalysts, the XRD there patterns was noof obviousthethe usedusedchange catalysts,catalysts, in characteristictherethere waswas nono diffraction obviousobvious changechange peaks, inin which characteristic showedthat diffraction the catalyst peaks, structure which retainedshowed itsthatthat original thethe catalystcatalyst patterns. strucstructureture retainedretained itsits original patterns.

Figure 5. XRD patterns of TS-1, used TS-1, S-1, and commercial TS-1. Figure 5. XRD patterns of TS-1,TS-1, used TS-1,TS-1, S-1,S-1, and commercial TS-1.TS-1.

2.2.4..4. ScanningScanning ElectronElectron MicroscopyMicroscopy (SEM)(SEM) andand TransmissionTransmission ElectronElectron MicroscopyMicroscopy (TEM)(TEM) 2.2.4. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) The SEM images are shown in Figure 6. The SEM images demonstrated that thethe TS--1 was uniform. The SEM images are shown in Figure6. The SEM images demonstrated that the TS-1 was uniform. The particles of TS--1 were rough--surface spheres of an average size of about 100–150 nm. The particle size The particles of TS-1 were rough-surface spheres of an average size of about 100–150 nm. The particle of commercial TS--1 was much larger than thatthat ofof TS--1:: about 500–600 nm. The size of some particles was size of commercial TS-1 was much larger than that of TS-1: about 500–600 nm. The size of some particles very big, which revealed the nonuniformity inin sizesize of commercial TS--1. The agglomeration of commercial was very big, which revealed the nonuniformity in size of commercial TS-1. The agglomeration of TS--1 was distinct, indicating an uneven distribution of catalyst particles. The TS--1 used fourfour timetimes after calcination displayed a similar morphologic structure toto itsits parent TS--1.

Catalysts 2018, 8, 49 6 of 14 commercial TS-1 was distinct, indicating an uneven distribution of catalyst particles. The TS-1 used four timesCatalysts after 2018, 8 calcination, x FOR PEER REVIEW displayed a similar morphologic structure to its parent TS-1. 6 of 14

Catalysts 2018, 8, x FOR PEER REVIEW 6 of 14

Figure 6. SEM images of TS-1 (a,b), TS-1 used four times (c,d), and commercial TS-1 (e,f). Figure 6. SEM images of TS-1 (a,b), TS-1 used four times (c,d), and commercial TS-1 (e,f).

The TEMFigure images 6. SEM of images TS-1, ofused TS- 1TS (a-,b1,), STS-1-,1 and used commercial four times ( cTS,d),- 1and are commercial shown in TSFigure-1 (e, f7.). From the Thehigh TEMresolution images TEM of images, TS-1, usedit could TS-1, be observed S-1, and that commercial the particles TS-1 of TS are-1 had shown an obvious in Figure hierarchical7. From the highstructure resolutionThe TEMand TEM a images rough images, surface.of TS it-1, could Theused particles beTS observed-1, S -of1, TSand- that1 commercialconsisted the particles of TSsponge-1 ofare- TS-1 likeshown irregular had inan Figure obviousaggregates 7. From hierarchical with the structureahigh size resolution and that aapproached rough TEM surface.images, 100– it15 Thecould0 nm particles be[39 observed,57]. The of TS-1that TEM the consisted imagesparticles of of S TS- sponge-like1 - showed1 had an thatobvious irregular S-1 hierarchical had aggregatesa less- withhierarchicalstructure a size that and structure approacheda rough andsurface. a 100–150slightly The particles larger nm [particle 39of ,TS57-].1 size consisted The relative TEM of tosponge images the TS-like- of1. Theirregular S-1 commercial showed aggregates that TS-1 S-1 withalso had a less-hierarchicalhada size a thathierarchical approached structure structure, and 100– a1 5 slightlybut0 nm the [ 39 largerparticle,57]. The particle size TEM and size images shape relative of was S- 1 to nonuniform. showed the TS-1. that The The S-1 commercial particle had a lesss of- TS-1 commercialhierarchical structure TS-1 were and irregular a slightly with larger no specificparticle shape.size relative In combin to theation TS-1. with The thecommercial SEM images, TS-1 also the also had a hierarchical structure, but the particle size and shape was nonuniform. The particles of commercialhad a hierarchical TS-1 had structure, a much larger but theparticle particle size sizethan andthe TS shape-1 prepared was nonuniform. in present work, The which particle wents of commercial TS-1 were irregular with no speciﬁc shape. In combination with the SEM images, the againstcommercial the diffusion TS-1 were of theirregular reactant with and noproducts specific [41 shape.]. The TEMIn combin imagesation of the with reused the SEMcatalyst images, indicated the commercialthatcommercial the TS-1 catalysts TS had-1 had still a amuch retainedmuch larger a shape particle particle resembling size size than than thethe TS the parent-1 TS-1prepared TS- prepared1, whichin present showed in presentwork, a which stability work, went of which wentmorphology.against against the the diffusion diffusion of the of reactant the reactant and products and products [41]. The [TEM41]. Theimages TEM of the images reused of catalyst the reused indicated catalyst indicatedthat the that catalysts the catalysts still retained still retained a shape a shaperesembling resembling the parent the parent TS-1, which TS-1, showed which showed a stability a stability of of morphology.morphology.

Figure 7. TEM images TS-1 (a,b), TS-1 used four times (c,d), S-1 (e,f), and commercial TS-1 (g,h).

2.2Figure.5. InductivelyFigure 7. TEM 7. TEM Coupled images images TS-1 Plasma TS- (1a (,ab ,(ICP)),b), TS-1 TS -and1 used used X- ray fourfour Photoelectron time timess (c (,cd,d),), S- S-11 (Spectroscopye, (fe),,f and), and commercial commercial (XPS) TS-1 TS-1 (g,h). ( g,h). The XRF analysis showed that TS-1 only had Si, Ti, and O elements, without potassium and 2.2.5. Inductively Coupled Plasma (ICP) and X-ray Photoelectron Spectroscopy (XPS) sodium, from the template of tetrapropylammonium hydroxide (TPAOH). ICP was performed to The XRF analysis showed that TS-1 only had Si, Ti, and O elements, without potassium and sodium, from the template of tetrapropylammonium hydroxide (TPAOH). ICP was performed to

CatalystsCatalysts 2018, 8, x 49 FOR PEER REVIEW 77 ofof 1414

analyze the Ti content of the catalyst. A pretreatment using HF to dissolve Si in a Teflon vessel was 2.2.5.applied, Inductively then aqua Coupled regia was Plasma used to (ICP) dissolve and all X-ray of the Photoelectron elements after Spectroscopy the evaporation (XPS) of HF. The liquid mixtureThe was XRF analyzed analysis by showed ICP to that quantify TS-1 onlythe content had Si, of Ti, Ti and. The O Ti elements, content withoutof the TS potassium-1 catalyst andwas sodium,detected fromto be 1.13 the templatewt.%, and of the tetrapropylammonium Ti content of the commercial hydroxide TS-1 (TPAOH).was detected ICP to was be 4.80 performed wt.%. The to analyzeTi 2p3/2 spectra the Ti contentare shown of thein Figure catalyst. 8. The A pretreatment peak at the binding using HF energy to dissolve (BE) at Siaround in a Teﬂon 460 eV vessel belonged was applied,to the tetrahedral then aqua regia Ti in was the usedframework, to dissolve and all the of the peak elements at the after BE atthe 458 evaporation eV turned of HF. out The to be liquid the mixtureextraframework was analyzed Ti in the by catalyst ICP to quantify[58–62]. As the shown content in ofFigure Ti. The 8, the Ti single content peak of theat the TS-1 BE catalyst at 460 eV was of detectedthe TS-1 catalyst to be 1.13 demonstrated wt.%, and that the Tialmost content only of framework the commercial Ti species TS-1 existed. was detected The obvious to be peak 4.80 at wt.%. 458 eV of the commercial TS-1 belonged to the extraframework TiO2. Table 1 lists the surface Ti content The Ti 2p3/2 spectra are shown in Figure8. The peak at the binding energy (BE) at around 460 eV belongedpercentage to detected the tetrahedral by XPS. Ti The in thecontent framework, of surface and Ti the species peak was at the far BE less at 458than eV the turned total Ti out species to be thefor extraframeworkTS-1 and commercial Ti in TS the-1, catalyst suggesting [58– 62that]. Asthe showncontent in of Figure surface8, Ti the species single w peakas much at the less BE than at 460 the eV bulk of thecontent. TS-1 The catalyst TS-1 demonstratedcatalyst almost that only almost had framework only framework Ti species Ti on species the surface existed.: about The obvious0.21 wt.%. peak It was at clear that the majority of Ti species of the commercial TS-1 dispersed in the extraframework—about 458 eV of the commercial TS-1 belonged to the extraframework TiO2. Table1 lists the surface Ti content percentage0.92 wt.%— detectedand the contrib by XPS.ution The at content 460 eV of from surface framew Ti speciesork Ti wasspecies far was less than0.34 wt.%, the total which Ti species was more for TS-1than andthe commercialTS-1 catalyst. TS-1, As suggestingshown in Table that the 1, contentthe surface of surface Ti content Ti species of the was TS- much1 kept less almost than stable the bulk in content.consecutive The runs. TS-1 catalystThis might almost be the only reason had framework for the stable Ti speciesactivity on of thethe surface:catalyst. aboutBased 0.21 on the wt.%. literature It was clear[49,50 that], the the extraframework majority of Ti species TiO2 was of the responsible commercial for TS-1 the dispersed decomposition in the extraframework—about of H2O2. Although the 0.92commercial wt.%—and TS-1 the contained contribution more atcontent 460 eV of from surface framework framework Ti species Ti species was than 0.34 the wt.%, TS- which1, the amount was more of thanextraframework the TS-1 catalyst. Ti species As was shown also in significant. Table1, the Thus, surface the decomposition Ti content of the of H TS-12O2 kepton commercial almost stable TS-1 inmight consecutive be serious, runs. resulting This in might a low be yield the reasonof phenol. for the stable activity of the catalyst. Based on the The resulting filtrate after reaction was analyzed by ICP to detect the leaching of Ti. The amount literature [49,50], the extraframework TiO2 was responsible for the decomposition of H2O2. Although theof the commercial leaching TS-1of Ti contained in the first more run content was 0.12 of surfacewt.%, and framework then almost Ti species no leaching than the of TS-1, Ti species the amount was detected. As no obvious change in the morphology of TS-1 was observed, the leaching of Ti was the of extraframework Ti species was also signiﬁcant. Thus, the decomposition of H2O2 on commercial TS-1reason might for the be reduction serious, resulting of the activity in a low of yieldthe catalyst of phenol. from the first run to the second.

FigureFigure 8. XPS spectra of TS TS-1-1 ( a)),, TS TS-1-1 used once ( b), TS TS-1-1 used f fourour times ( c), and commercial TS TS-1-1 ( d).

Catalysts 2018, 8, 49 8 of 14

Table 1. Surface Ti content of catalysts.

Sample 458 eV (wt.%) 460 eV (wt.%) Total (wt.%) TS-1 - 0.21 0.21 Catalysts 2018, 8, x FOR PEER REVIEW 8 of 14 TS-1(used once) - 0.26 0.26 TS-1(used four times) - 0.26 0.26 commercial TS-1Table 0.92 1. Surface Ti content of catalysts. 0.34 1.26

Sample 458 eV (wt.%) 460 eV (wt.%) Total (wt.%) The resulting filtrate afterTS-1 reaction was analyzed- by ICP0.21 to detect the leaching0.21 of Ti. The amount of the leaching of TiTS in- the1(used first once) run was 0.12 wt.%,- and then0.26 almost no leaching0.26 of Ti species was detected. As no obvious change in the morphology of TS-1 was observed, the leaching of Ti was the TS-1(used four times) - 0.26 0.26 reason for the reduction of the activity of the catalyst from the first run to the second. commercial TS-1 0.92 0.34 1.26

2.2.6. N2 Adsorption-Desorption Isotherms and Pore Size Distribution Curves 2.2.6. N2 Adsorption-Desorption Isotherms and Pore Size Distribution Curves The N2 adsorption-desorption isotherms of TS-1, used TS-1, S-1, and commercial TS-1 are shown The N2 adsorption-desorption isotherms of TS-1, used TS-1, S-1, and commercial TS-1 are shown in Figure9a. The gentle uptake at P/P 0 < 0.05, and almost no adsorption at intermediate relative in Figure 9a. The gentle uptake at P/P0 < 0.05, and almost no adsorption at intermediate relative pressures, suggested the presence of micropores. The existence of a hysteresis loop at P/P0 > 0.9 could bepressures, attributed suggested to the interparticle the presence adsorption of micropor withines. The the existence voids formed of a hysteresis between loop the at catalyst P/P0 > particles,0.9 could whichbe attributed indicated to thatthe interparticle the samples adsorption consisted of within both microporous the voids formed and mesoporous between the channels catalyst [particles,19,42,63]. Thewhich commercial indicated TS-1that the showed samples a type-I consisted isotherm, of both typical microporous of microporous and mesoporous material, channels and the hysteresis[19,42,63]. loopsThe commercial in the nitrogen TS-1 isotherm showed indicateda type-I isotherm, the existence typical of minorityof microporous mesopores material, [38,64 ,and65]. the The hysteresis pore size distributionloops in the curvesnitrogen showed isotherm that indicated the pore the size existence of the TS-1 of wasminority concentrated mesopores at15–18 [38,64 Å,,65 and]. The mesopores pore size existeddistribution obviously, curves as showed shown that in Figure the pore9b. Thesize commercialof the TS-1 was TS-1 concentrated displayed a sharpat 15–1 distribution8 Å , and mesopores of pore widthexisted at obviously, 15 Å. Table as2 shownlists the in textural Figure 9b. parameters The commercial of the samples. TS-1 displayed The TS-1 a sharp catalyst distribution had a relatively of pore largewidth adsorption at 15 Å . Table average 2 lists pore the textural size of 23.0 parameters Å and a of Brunauer–Emmett–Teller the samples. The TS-1 catalyst (BET) had surface a relatively area of 476large m 2adsorptiong−1. The commercial average pore TS-1 size had of a 23.0 smaller Å and adsorption a Brunauer average–Emmett pore–Teller size of ( 17.0BET) Å surface and a BET area surface of 476 2 −1 aream g of. T 411he commercial m2g−1. The TS TS-1-1 had catalyst a smaller had aadsorption greater microporous average pore volume size of and17.0 mesoporousÅ and a BET volume surface 2 −1 thanarea theof 411 commercial m g . The TS-1, TS-1 and catalyst the commercial had a greater TS-1, microporous with a microporous volume and volume mesoporous of 0.084 cmvolume3g−1, than was 3 −1 muchthe commercial smaller than TS-1, the and TS-1 the catalyst commercial of 0.17 TS cm-1, 3withg−1 ,a which microporous might bevolu causedme of by0.084 the cm extraframeworkg , was much 3 −1 Tismaller species. than the TS-1 catalyst of 0.17 cm g , which might be caused by the extraframework Ti species.

FigureFigure 9.9. N22 adsorptionadsorption-desorption-desorption isothermsisotherms ((aa)) and and pore pore diameter diameter distribution distribution (b ()b of) of TS-1, TS-1 used, used TS-1, TS- S-1,1, S- and1, and commercial commercial TS-1. TS-1.

Table 2. Structural properties of the samples.

Sample SBET (m2g−1) Vmic (cm3g−1) Vmeso (cm3g−1) ΦPa (Å) TS-1 476 0.17 0.16 23.0 TS-1(used once) 480 0.17 0.17 23.4 TS-1(used four times) 460 0.16 0.16 23.1 S-1 458 0.16 0.31 36.8 commercial TS-1 411 0.084 0.11 17.0

Catalysts 2018, 8, 49 9 of 14

Table 2. Structural properties of the samples.

2 −1 3 −1 3 −1 a Sample SBET (m g ) Vmic (cm g ) Vmeso (cm g ) ΦP (Å) TS-1 476 0.17 0.16 23.0 TS-1(used once) 480 0.17 0.17 23.4 TS-1(used four times) 460 0.16 0.16 23.1 S-1 458 0.16 0.31 36.8 commercial TS-1 411 0.084 0.11 17.0

The maximum molecule radius of benzene, phenol, catechol, hydroquinone, and benzoquinone was 5.0 Å, 5.7 Å, 5.7 Å, 6.5 Å, and 5.5 Å, respectively (calculated value from Gaussian 09). The Ti species of the samples were mainly distributed within pores detected by XPS, so the added benzene could be adsorbed well in the hydrophobic catalyst channel [61]. The TS-1 catalyst with a relatively large pore size and pore volume permitted many more benzene molecules to react with as many Ti active sites as possible, and the products could leave simultaneously, which greatly accelerated the reaction rate. However, the larger pore size provided sufﬁcient space for phenol over-oxidation to DHB and benzoquinone [66], which led to a decrease in the selectivity of phenol. The pore volume and pore size of the commercial TS-1 was relatively small, which might inhibit the adsorption of benzene and the diffusion of products. Consequently, the other benzene molecules could not enter into the blocked channel simultaneously, resulting in a decrease in catalytic activity. The TS-1 reused three times almost had no changes in the N2 adsorption-desorption isotherms and the pore size distribution curve. From the textural parameters of the used TS-1, the values of the BET surface area, pore volume, and adsorption average pore size had almost no changes compared with the parent TS-1. The N2 adsorption-desorption isotherms and pore size distributions of S-1 are shown in Figure9, and the textural parameters are listed in Table2, which indicated that the pore size was mainly distributed in the mesoporous range [67], and the adsorption average pore width of S-1 was around 40.8 Å. According to the characterization of the samples, almost all of the Ti species of the TS-1 catalyst entered into the framework without post-treatment. S-1 had a similar structure to the TS-1 catalyst without Ti species. The commercial TS-1 contained Ti species, but the majority of them were located at the extraframework, and had some structural differences from the TS-1 catalyst. Based on the results of the catalytic activity, it was evident that the framework Ti species were the active sites for the hydroxylation of benzene. The TS-1 catalyst with a relatively large pore size and pore volume permitted the benzene molecules to react with most of the Ti species within the pores, which shortened the reaction time. The amount of extraframework TiO2 dispersed on the surface of the commercial TS-1 was more than that of the framework Ti species, which might decrease the catalytic activity by an ineffective decomposition of H2O2.

3. Experimental Section

3.1. Materials

The tetraethyl orthosilicate, isopropanol, sodium chloride (NaCl), hydrogen peroxide (H2O2, 30%), benzene, phenol, chlorobenzene, and ethyl ether were purchased from Chengdu Kelong Chemical Regent Co., LTD., Chengdu, China. The tetrapropylammonium hydroxide (TPAOH, 25%) was obtained from Shanghai Drﬁnechem Fine Chemicals Co., LTD., Shanghai, China. The titanium trichloride (TiCl3, 15–20 wt.%) came from the Tianjin Damao Reagent factory. The catechol (CAT, 99%), hydroquinone (HQ, 99%), and benzoquinone (BQ, 99%) were obtained from TCI, Shanghai, China. The commercial TS-1 was acquired from Nanjing XFNANO Materials Tech Co., LTD., Nanjing, China. All of the materials were used directly without further puriﬁcation. Catalysts 2018, 8, 49 10 of 14

3.2. Methods

3.2.1. Catalyst Preparation The TS-1 catalyst was prepared by a hydrothermal synthesis method which was similar to that in our previous work [32]. Namely, 20 g of isopropanol was ﬁrst dropwise added into 45 g of tetraethyl orthosilicate with stirring, followed by the addition of 20 g of water. After stirring for 30 min, 60 g of aqueous solution of TPAOH (20%) was dropwise added to the mixture. Then, 3.5 g of titanium trichloride (15.0 wt.%) diluted by 10 g of H2O was dropwise added to the mixture, and was then replenished with 60 g of water. Then, we heated the solution in a water bath at 55 ◦C for 1 h and at 77 ◦C for 7 h. In this heating step, water was dropwise added continuously to keep the total volume invariable. During the whole process, a magnetic stirring apparatus was used to keep the solution mixed well. The resulting solution was cooled down, kept overnight, and crystallized. The crystallization was carried out in a PTFE-lined stainless steel autoclave at 175 ◦C for 7 days. The white solid product was ﬁltered and washed with deionized water and dried at 80 ◦C for more than one day. The dried white product was calcined at 550 ◦C for 10 h to remove the template (TPAOH). Finally, the TS-1 catalyst was obtained.

3.2.2. Catalyst Characterization The X-ray diffraction (XRD) measurements were performed on an Empyrean Diffractometer (PANalytical B.V., Almelo, The Netherlands) with a Cu-Kα radiation source. The data over a 2θ scanning range from 5◦ to 60◦ were collected at a step size of 0.02◦. The surface morphology of the samples was characterized by scanning electron microscopy (JSM-7500F, JEOL, Tokyo, Japan) set at 5 kV. Transmission electron microscopy images of the samples were performed on an FEI company Tecnai G220 Twin instrument (FEI, Hillsboro, OR, USA) at an acceleration voltage of 200 kV. DR UV-Vis spectra of the samples were obtained with a Hitachi U-4100 spectrometer (Hitachi, Tokyo, Japan) equipped with a reﬂectance attachment. BaSO4 was used as the reference. Fourier transform infrared (FT-IR) spectra of the samples were collected at a 2 cm−1 resolution on a Nicolet Nexus 670 FT-IR spectrometer (Thermo Fisher Scientiﬁc, Waltham, MA, USA) equipped with an MCT detector. N2 adsorption-desorption isotherms of TS-1 were measured at 77 K on a Micromeritics Tristar 3020 instrument (Micromeritics, Atlanta, GA, USA). The catalysts were degassed at 300 ◦C under vacuum for 2 h before N2 adsorption-desorption. The Brunauer–Emmett–Teller (BET) method was applied to obtain the surface areas. The Barrett–Joyner–Halenda (BJH) method was applied to obtain pore volumes and average pore diameters. Inductively coupled plasma (ICP) was used to analyze the Ti content of the samples and was performed on a VG PQExCell instrument (TJA, Boston, MA, USA). X-ray photoelectron spectroscopy (XPS) was performed on an AXIS Ultra DLD (KRATOS) spectrometer (Shimadzu, Kyoto, Japan).

3.2.3. Catalytic Activity Test The reaction was carried out in a 50 mL flask with the integration of a reflux condenser. In a typical run, a certain amount of catalyst was added, then 5.6 mmol benzene, 40 mL deionized water, and 0.80 mL H2O2 (30%) were introduced into the reaction flask in order. The flask was heated to 70 ◦C to proceed the reaction for 45 min. After the reaction system was cooled to room temperature, the resulting mixture was separated by filtration. NaCl was added into the filtrate until saturation, then the salt-added liquid was extracted by ethyl ether for four times. Standard substances were employed to test the extraction rate. A certain amount of standard substance was dissolved in a given volume of water, and the same extraction procedure as that for the extraction of products was used. Then, the obtained standard substance contained in ether solution was analyzed by GC to get the extraction rate of the substance. The extracted liquid phase was identified by GC-MS (Agilent, 5973 Network 6890N, Santa Clara, CA, USA) Catalysts 2018, 8, 49 11 of 14 and quantified by GC (a Fuli 9750 equipped with a Flame Ionization Detector (FID) and a DB-5 column) using chlorobenzene as an internal standard. The yield and selectivity of the products, including phenol (PH), catechol (CAT), hydroquinone (HQ), benzoquinone (BQ), and DHB (CAT + HQ) were calculated as the following:

( ) = Mole of amount of phenol Yield of phenol mol % Initial mole amount of benzene×extraction rate (extraction rate of phenol 72%)

( ) = Mole of amount of catechol Yield of catechol mol % Initial mole amount of benzene×extraction rate (extraction rate of catechol 72%) ( ) = Mole of amount of hydroquinone Yield of hydroquinone mol % Initial mole amount of benzene×extraction rate (extraction rate of hydroquinone 65%) ( ) = Mole of amount of benzoquinone Yield of benzoquinone mol % Initial mole amount of benzene×extraction rate (extraction rate of benzoquinone 95%) Mole amount of phenol Selectivity (%) = Mole amounts of phenol + DHB + BQ Mole amount of phenol + DHB + BQ Conversion of benzene (%) = Initial mole amount of benzene Mole of amount of phenol TOF of phenol h−1 = Mole amount of Ti × reaction time (h)

4. Conclusions In summary, TS-1 as the catalyst for the hydroxylation of benzene had effective catalytic activity under mild reaction conditions. Under the optimized conditions, the yield of phenol reached 39% and the selectivity of phenol reached 72%. The process of reaction was simple and fast, with a short reaction time, only 45 min, and a low concentration of H2O2 of about 0.20 mol/L. The TS-1 catalyst almost only had framework Ti species, which were the active sites for the hydroxylation of benzene. The surface-dispersed extraframework TiO2 contributed to the decrease of catalytic activity. The relatively large pore size might enhance the reaction rate. The TS-1 catalyst showed a relatively good reusability, as there was only a slight decrease from the ﬁrst run.

Acknowledgments: This work was financially supported by the the NSFCC (No. 21172157, No. 21372167) and Sichuan province (2016JY0181). The characterization of the residue from the Analytical and Testing Center of Sichuan University are greatly appreciated. We would like to thank Yunfei Tian of the Analytical and Testing Center of Sichuan University for the XPS experiments. Author Contributions: G.L. and C.H. conceived and designed the experiments; Y.L., J.X., and C.P. performed the experiments; and Y.L., G.L., and C.H. analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors were not involved in the design of the study, the collection, the analyses, the interpretation of the data, the writing of the manuscript, nor the decision to publish the results.

References

1. Yamanaka, H.; Hamada, R.; Nibuta, H.; Nishiyama, S.; Tsuruya, S. Gas-phase catalytic oxidation of benzene over Cu-supported ZSM-5 catalysts: An attempt of one-step production of phenol. J. Mol. Catal. A 2002, 178, 89–95. [CrossRef] 2. Schmidt, R.J. Industrial catalytic processes-phenol production. Appl. Catal. A 2005, 280, 89–103. [CrossRef] 3. Lemke, K.; Ehrich, H.; Lohse, U.; Berndt, H.; Jähnisch, K. Selective hydroxylation of benzene to phenol over supported vanadium oxide catalysts. Appl. Catal. A 2003, 243, 41–51. [CrossRef] 4. Ehrich, H.; Berndt, H.; Pohl, M.M.; Jähnisch, K.; Baerns, M. Oxidation of benzene to phenol on supported Pt-VOx and Pd-VOx catalysts. Appl. Catal. A 2002, 230, 271–280. [CrossRef] 5. Fukuzumi, S.; Ohkubo, K. One-step selective hydroxylation of benzene to phenol. Asian J. Org. Chem. 2015, 4, 836–845. [CrossRef] Catalysts 2018, 8, 49 12 of 14

6. Choi, J.S.; Kim, T.H.; Saidutta, M.B.; Sung, J.S.; Kim, K.I.; Jasra, R.V.; Song, S.D.; Rhee, Y.W. Benzene hydroxylation to phenol catalyzed by transition metals supported on MCM-41 and activated carbon. J. Ind. Eng. Chem. 2004, 10, 445–453. 7. Balducci, L.; Bianchi, D.; Bortolo, R.; D’Aloisio, R.; Ricci, M.; Tassinari, R.; Ungarelli, R. Direct oxidation of benzene to phenol with hydrogen peroxide over a modiﬁed titanium silicalite. Angew. Chem. 2003, 42, 4937–4940. [CrossRef][PubMed] 8. Peng, J.; Shi, F.; Gu, Y.; Deng, Y. Highly selective and green aqueous–ionic liquid biphasic hydroxylation of benzene to phenol with hydrogen peroxide. Green Chem. 2003, 5, 224–226. [CrossRef] 9. Niwa, S.S.; Eswaramoorthy, M.; Nair, J.; Raj, A.; Itoh, N.; Shoji, H.; Namba, T.; Mizukami, F. A one-step conversion of benzene to phenol with a palladium membrane. Science 2002, 295, 105–107. [CrossRef] [PubMed] 10. Louis, B.; Reuse, P.; Kiwi-Minsker, L.; Renken, A. Synthesis of ZSM-5 coatings on stainless steel grids and

their catalytic performance for partial oxidation of benzene by N2O. Appl. Catal. A 2001, 210, 103–109. [CrossRef] 11. Gu, Y.Y.; Zhao, X.H.; Zhang, G.R.; Ding, H.M.; Shan, Y.K. Selective hydroxylation of benzene using dioxygen activated by vanadium–copper oxide catalysts supported on SBA-15. Appl. Catal. A 2007, 328, 150–155. [CrossRef]

12. Murata, K.; Yanyong, R.; Inaba, M. Effects of vanadium supported on ZrO2 and sulfolane on the synthesis of phenol by hydroxylation of benzene with oxygen and acetic acid on palladium catalyst. Catal. Lett. 2005, 102, 143–147. [CrossRef] 13. Selli, E.; Rossetti, I.; Meloni, D.; Sini, F.; Forni, L. Effect of surface acidity on the behaviour of Fe-MFI catalysts for benzene hydroxylation to phenol. Appl. Catal. A 2004, 262, 131–136. [CrossRef] 14. Taramasso, M.; Perego, G.; Notari, B. Preparation of Porous Crystalline Synthetic Material Comprised of Silicon and Titanium Oxides. U.S. Patent No 4,410,501, 18 October 1983. 15. Bianchi, D.; D’Aloisio, R.; Bortolo, R.; Ricci, M. Oxidation of mono- and bicyclic aromatic compounds with hydrogen peroxide catalyzed by titanium silicalites TS-1 and TS-1b. Appl. Catal. A 2007, 327, 295–299. [CrossRef] 16. Zou, H.; Sun, Q.; Fan, D.; Fu, W.; Liu, L.; Wang, R. Facile synthesis of yolk/core-shell structured TS-1@mesosilica composites for enhanced hydroxylation of phenol. Catalysts 2015, 5, 2134–2146. [CrossRef] 17. Fang, Y.; Hu, H. Mesoporous TS-1: Nanocasting synthesis with CMK-3 as template and its performance in catalytic oxidation of aromatic thiophene. Catal. Commun. 2007, 8, 817–820. [CrossRef] 18. Liu, T.; Wang, L.; Wan, H.; Guan, G. A magnetically recyclable TS-1 for ammoximation of cyclohexanone. Catal. Commun. 2014, 49, 20–24. [CrossRef] 19. Gao, X.; An, J.; Gu, J.; Li, L.; Li, Y. A green template-assisted synthesis of hierarchical TS-1 with excellent catalytic activity and recyclability for the oxidation of 2,3,6-trimethylphenol. Microporous Mesoporous Mater. 2017, 239, 381–389. [CrossRef] 20. Schmidt, I.; Krogh, A.; Wienberg, K.; Carlsson, A.; Brorson, M.; Jacobsen, C.J.H. Catalytic epoxidation of alkenes with hydrogen peroxide over first mesoporous titanium-containing zeolite. Chem. Commun. 2000, 2157–2158. [CrossRef] 21. Serrano, D.P.; Sanz, R.; Pizarro, P.; Moreno, I. Tailoring the properties of hierarchical TS-1 zeolite synthesized from silanized protozeolitic units. Appl. Catal. A 2012, 435–436, 32–42. [CrossRef] 22. Huang, Y.; Wang, K.; Dong, D.; Li, D.; Hill, M.R.; Hill, A.J.; Wang, H. Synthesis of hierarchical porous zeolite nay particles with controllable particle sizes. Microporous Mesoporous Mater. 2010, 127, 167–175. [CrossRef] 23. Xu, J.; Liu, H.; Yang, R.; Li, G.; Hu, C. Hydroxylation of benzene by activated carbon catalyst. Chin. J. Catal. 2012, 33, 1622–1630. [CrossRef] 24. Zhang, L.; Liu, H.; Li, G.; Hu, C. Continuous flow reactor for hydroxylation of benzene to phenol by hydrogen peroxide. Chin. J. Chem. Phys. 2012, 25, 585–591. [CrossRef] 25. Wcław, A.; Nowińska,K.; Schwieger, W. Benzene to phenol oxidation over iron exchanged zeolite ZSM-5. Appl. Catal. A 2004, 270, 151–156. [CrossRef] 26. Shahid, A.; Lopez-Orozco, S.; Marthala, V.R.; Hartmann, M.; Schwieger, W. Direct oxidation of benzene to phenol over hierarchical ZSM-5 zeolites prepared by sequential post synthesis modification. Microporous Mesoporous Mater. 2016, 237, 151–159. [CrossRef] Catalysts 2018, 8, 49 13 of 14

27. Sato, K.; Hanaoka, T.; Niwa, S.; Stefan, C.; Namba, T.; Mizukami, F. Direct hydroxylation of aromatic compounds by a palladium membrane reactor. Catal. Today 2005, 104, 260–266. [CrossRef] 28. Wang, X.; Guo, Y.; Zhang, X.; Wang, Y.; Liu, H.; Wang, J.; Qiu, J.; Yeung, K.L. Catalytic properties of benzene hydroxylation by TS-1 ﬁlm reactor and Pd–TS-1 composite membrane reactor. Catal. Today 2010, 156, 288–294. [CrossRef] 29. Ye, S.; Hamakawa, S.; Tanaka, S.; Sato, K.; Esashi, M.; Mizukami, F. A one-step conversion of benzene to phenol using mems-based Pd membrane microreactors. Chem. Eng. J. 2009, 155, 829–837. [CrossRef] 30. Hu, L.; Wang, C.; Ye, L.; Wu, Y.; Yue, B.; Chen, X.; He, H. Direct hydroxylation of benzene to phenol using

H2O2 as an oxidant over vanadium-containing mesoporous carbon catalysts. Appl. Catal. A 2015, 504, 440–447. [CrossRef] 31. Chammingkwan, P.; Hoelderich, W.F.; Mongkhonsi, T.; Kanchanawanichakul, P. Hydroxylation of benzene over TS-PQ™ catalyst. Appl. Catal. A 2009, 352, 1–9. [CrossRef] 32. Ye, X.; Cui, Y.; Qiu, X.; Wang, X. Selective oxidation of benzene to phenol by Fe-CN/TS-1 catalysts under visible light irradiation. Appl. Catal. B 2014, 152–153, 383–389. [CrossRef]

33. Chen, X.; Zhang, J.; Fu, X.; Antonietti, M.; Wang, X. Fe-gC3N4-catalyzed oxidation of benzene to phenol using hydrogen peroxide and visible light. J. Am. Chem. Soc. 2009, 131, 11658–11659. [CrossRef][PubMed] 34. Liu, H.; Fu, Z.; Yin, D.; Yin, D.; Liao, H. A novel micro-emulsion catalytic system for highly selective hydroxylation of benzene to phenol with hydrogen peroxide. Catal. Commun. 2005, 6, 638–643. [CrossRef] 35. Feng, S.; Pei, S.; Yue, B.; Ye, L.; Qian, L.; He, H. Synthesis and characterization of V-HMS employed for catalytic hydroxylation of benzene. Catal. Lett. 2009, 131, 458–462. [CrossRef] 36. Tang, Y.; Zhang, J. Direct oxidation of benzene to phenol catalyzed by vanadium substituted heteropolymolybdic acid. Transit. Met. Chem. 2006, 31, 299–305. [CrossRef] 37. Wang, X.; Zhang, T.; Li, B.; Yang, Q.; Jiang, S. Efﬁcient hydroxylation of aromatic compounds catalyzed by

an iron(II) complex with H2O2. Appl. Organomet. Chem. 2014, 28, 666–672. [CrossRef] 38. Xia, S.; Yu, T.; Liu, H.; Li, G.; Hu, C. One step C–N bond formation from alkylbenzene and ammonia over Cu-modified TS-1 zeolite catalyst. Catal. Sci. Technol. 2014, 4, 3108–3119. [CrossRef] 39. Yu, T.; Zhang, Q.; Xia, S.; Li, G.; Hu, C. Direct amination of benzene to aniline by reactive distillation method over copper doped hierarchical TS-1 catalyst. Catal. Sci. Technol. 2014, 4, 639–647. [CrossRef] 40. Nan, M.; Luo, Y.; Li, G.; Hu, C. Improvement of the selectivity to aniline in benzene amination over Cu/TS-1 by potassium. RSC Adv. 2017, 7, 21974–21981. [CrossRef] 41. Zhang, F.; Shang, H.; Jin, D.; Chen, R.; Xing, W. High efficient synthesis of methyl ethyl ketone oxime from ammoximation of methyl ethyl ketone over TS-1 in a ceramic membrane reactor. Chem. Eng. Process. 2017, 116, 1–8. [CrossRef] 42. Xin, H.; Zhao, J.; Xu, S.; Li, J.; Zhang, W.; Guo, X.; Hensen, E.J.M.; Yang, Q.; Li, C. Enhanced catalytic oxidation by hierarchically structured TS-1 zeolite. J. Phys. Chem. C 2010, 114, 6553–6559. [CrossRef] 43. Adedigba, A.; Sankar, G.; Catlow, C.R.A.; Du, Y.; Xi, S.; Borgna, A. On the synthesis and performance of hierarchical nanoporous TS-1 catalysts. Microporous Mesoporous Mater. 2017, 244, 83–92. [CrossRef] 44. Wang, X.; Li, G.; Wang, W.; Jin, C.; Chen, Y. Synthesis, characterization and catalytic performance of hierarchical TS-1 with carbon template from sucrose carbonization. Microporous Mesoporous Mater. 2011, 142, 494–502. [CrossRef] 45. Fan, W.; Duan, R.G.; Yokoi, T.; Wu, P.; Kubota, Y.; Tatsumi, T. Synthesis, crystallization mechanism, and catalytic properties of titanium-rich TS-1 free of extraframework titanium species. J. Am. Chem. Soc. 2008, 130, 10150–10164. [CrossRef][PubMed] 46. Xiong, G.; Cao, Y.; Guo, Z.; Jia, Q.; Tian, F.; Liu, L. The roles of different titanium species in TS-1 zeolite in propylene epoxidation studied by in situ UV raman spectroscopy. Phys. Chem. Chem. Phys. 2016, 18, 190–196. [CrossRef][PubMed] 47. Wang, L.; Xiong, G.; Su, J.; Li, P.; Guo, H. In situ UV raman spectroscopic study on the reaction intermediates for propylene epoxidation on TS-1. J. Phys. Chem. C 2012, 116, 9122–9131. [CrossRef] 48. Petrini, G.; Cesana, A.; Alberti, G.D.; Genoni, F.; Leofanti, G.; Padovan, M.; Paparatto, G.; Roffia, P. Deactivation phenomena on Ti-silicalite. Stud. Surf. Sci. Catal. 1991, 68, 761–766. 49. Su, J.; Xiong, G.; Zhou, J.; Liu, W.; Zhou, D.; Wang, G.; Wang, X.; Guo, H. Amorphous Ti species in titanium silicalite-1: Structural features, chemical properties, and inactivation with sulfosalt. J. Catal. 2012, 288, 1–7. [CrossRef] Catalysts 2018, 8, 49 14 of 14

50. Huybrechts, D.R.C.; Buskens, P.L.; Jacobs, P.A. Physicochemical and catalytic properties of titanium silicalites. J. Mol. Catal. 1992, 71, 129–147. [CrossRef] 51. Wang, B.; Lin, M.; Peng, X.; Zhu, B.; Shu, X. Hierarchical TS-1 synthesized effectively by post-modiﬁcation with tpaoh and ammonium hydroxide. RSC Adv. 2016, 6, 44963–44971. [CrossRef] 52. Wu, G.; Xiao, J.; Zhang, L.; Wang, W.; Hong, Y.; Huang, H.; Jiang, Y.; Li, L.; Wang, C. Copper-modiﬁed TS-1 catalyzed hydroxylation of phenol with hydrogen peroxide as the oxidant. RSC Adv. 2016, 6, 101071–101078. [CrossRef]

53. Song, W.; Zuo, Y.; Xiong, G.; Zhang, X.; Jin, F.; Liu, L.; Wang, X. Transformation of SiO2 in titanium silicalite-1/SiO2 extrudates during tetrapropylammonium hydroxide treatment and improvement of catalytic properties for propylene epoxidation. Chem. Eng. J. 2014, 253, 464–471. [CrossRef] 54. Hu, Q.; Dou, B.; Tian, H.; Li, J.; Li, P.; Hao, Z. Mesoporous silicalite-1 nanospheres and their properties of adsorption and hydrophobicity. Microporous Mesoporous Mater. 2010, 129, 30–36. [CrossRef]

55. Yu, T.; Yang, R.; Xia, S.; Li, G.; Hu, C. Direct amination of benzene to aniline with H2O2 and NH3·H2O over Cu/SiO2 catalyst. Catal. Sci. Technol. 2014, 4, 3159–3167. [CrossRef] 56. Du, Q.; Guo, Y.; Duan, H.; Li, H.; Chen, Y.; Liu, H. Synthesis of hierarchical TS-1 zeolite via a novel three-step crystallization method and its excellent catalytic performance in oxidative desulfurization. Fuel 2017, 188, 232–238. [CrossRef]

57. Sanz, R.; Serrano, D.P.; Pizarro, P.; Moreno, I. Hierarchical TS-1 zeolite synthesized from SiO2 TiO2 xerogels imprinted with silanized protozeolitic units. Chem. Eng. J. 2011, 171, 1428–1438. [CrossRef]

58. Dai, J.; Zhong, W.; Yi, W.; Liu, M.; Mao, L.; Xu, Q.; Yin, D. Bifunctional H2WO4/TS-1 catalysts for direct conversion of cyclohexane to adipic acid: Active sites and reaction steps. Appl. Catal. B 2016, 192, 325–341. [CrossRef] 59. Wang, B.; Lin, M.; Zhu, B.; Peng, X.; Xu, G.; Shu, X. The synthesis, characterization and catalytic activity of the hierarchical TS-1 with the intracrystalline voids and grooves. Catal. Commun. 2016, 75, 69–73. [CrossRef] 60. Lee, W.S.; Cem Akatay, M.; Stach, E.A.; Ribeiro, F.H.; Nicholas Delgass, W. Enhanced reaction rate for

gas-phase epoxidation of propylene using H2 and O2 by Cs promotion of Au/TS-1. J. Catal. 2013, 308, 98–113. [CrossRef]

61. Kumar, R.; Bhaumik, A. Triphase, solvent-free catalysis over the TS-1/H2O2 system in selective oxidation reactions. Microporous Mesoporous Mater. 1998, 21, 497–504. [CrossRef] 62. Bhaumik, A.; Mukherjee, P.; Kumar, R. Triphase catalysis over titanium–silicate molecular sieves under solvent-free conditions: I. Direct hydroxylation of benzene. J. Catal. 1998, 178, 101–107. [CrossRef] 63. Zhong, W.; Tao, Q.; Jing, D.; Mao, L.; Xu, Q.; Zou, G.; Liu, X.; Yin, D.; Zhao, F. Visible-light-responsive sulfated vanadium-doped TS-1 with hollow structure: Enhanced photocatalytic activity in selective oxidation of cyclohexane. J. Catal. 2015, 330, 208–221. [CrossRef] 64. Tekla, J.; Tarach, K.; Olejniczak, Z.; Girman, V.; Góra-Marek, K. Effective hierarchization of TS-1 and its catalytic performance in cyclohexene epoxidation. Microporous Mesoporous Mater. 2016, 233, 16–25. [CrossRef] 65. Zuo, Y.; Song, W.; Dai, C.; He, Y.; Wang, M.; Wang, X.; Guo, X. Modiﬁcation of small-crystal titanium silicalite-1 with organic bases: Recrystallization and catalytic properties in the hydroxylation of phenol. Appl. Catal. A 2013, 453, 272–279. [CrossRef] 66. Xiao, F.; Han, Y.; Yu, Y.; Meng, X.; Yang, M.; Wu, S. Hydrothermally stable ordered mesoporous titanosilicates with highly active catalytic sites. J. Am. Chem. Soc. 2010, 33, 888–889. [CrossRef] 67. Feng, X.; Sheng, N.; Liu, Y.; Chen, X.; Chen, D.; Yang, C.; Zhou, X. Simultaneously enhanced stability

and selectivity for propene epoxidation with H2 and O2 on Au catalysts supported on nano-crystalline mesoporous TS-1. ACS Catal. 2017, 7, 2668–2675. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).